1. Field of the Invention
The present invention relates to hydrogen sulfide scavengers and methods of making and using the scavengers. More particularly, the invention relates to the increasing the capacity of a scavenger to remove sulfides from gaseous or liquid hydrocarbon streams, aqueous systems or mixture thereof.
2. Background of the Related Art
Hydrocarbons are typically found as gases or liquids located in geological formations beneath the earth's surface. These hydrocarbons may be produced by drilling a well into the formations. Gaseous hydrocarbons, typically referred to as natural gas, are frequently found to contain water, carbon dioxide and sulfides, such as hydrogen sulfides and mercaptans. Natural gas containing sulfides is referred to as being "raw" or "sour," due to its strong malodorous smell. However, the concentration of sulfides in natural gas will vary from formation to formation and perhaps over time. For example, the methane content of natural gas will typically vary between 45 and 96 percent and the hydrogen sulfide content will typically vary between 0.1 and 150,000 parts per million (ppm).
For many applications, it is desirable to remove some or substantially all the sulfides from the gas. For example, since hydrogen sulfide in the presence of water is corrosive to steel, it is desirable to removed substantially all of the hydrogen sulfide from natural gas before its use and preferably before transporting the gas or processing the gas in oil field equipment, pipelines and refineries. As a result, many pipeline specifications limit the amount of hydrogen sulfide to less than 0.25 grains per 100 cubic feet of gas. Accordingly, removal of hydrogen sulfide from hydrocarbons, such as natural gas, crude oil and refined oil products, is an important concern. Further, because of the large quantities of hydrocarbon materials that contain sulfides, it is important to have a means of removal that does not harm the environment and is economically efficient.
This removal is conventionally accomplished using gas processing plants which are typically located in the field and vary in size from small units located at each wellhead to larger, centrally located plants servicing multiple wells. These plants may be nothing more than a single bubble column or a multi-stage process involving continuously regenerable reagents.
These gas processing plants are typically provided with a solution containing a sulfide scavenging component which contacts the natural gas stream and reacts with the sulfides contained in the gas. Scavenging components, otherwise known as scavengers, that have been used to date have included formaldehyde, sodium nitrite, triazines and imines. However, certain of these scavengers have proven unsatisfactory for a variety of reasons. For example, formaldehyde has been found to be carcinogenic and requires careful handling and expensive disposal.
In recent years, the most widely used scavengers have been the triazines, particularly those formed by the reaction of formaldehyde and monoethanolamine. The popularity of this triazine scavenger is primarily due to its good scavenging capacity and low cost. However, triazine solutions may contain free formaldehyde if the triazine forming reaction is performed with excess formaldehyde. Furthermore, the reaction of triazines with the sulfides will lead to the formation of solids if the triazine solution is used to exhaustion. These solids represent a significant disposal problem and must be carefully cleaned from the scavenging process vessels.
Despite the predominant use of triazine scavengers, there remains a need for a sulfide scavenger solutions and processes that operate efficiently and effectively. It would be desirable if the scavenger solution and process reduced or eliminated solids formation and eliminated the use of toxic chemicals.